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Stack Molding

Double Your Machine Output

Compared to conventional single-face molds, stack molds can virtually double the output of an injection molding machine by distributing the plastic melt into two or more separate mold parting surfaces.

Injection Mold

Stack mold hot runner system in a tandem mold for large appliance with metal insert molding featuring a Rheo-Pro® iVG™ internal valve gate melt transfer

What is a Stack Mold?
The defining characteristic of a stack mold are the two (or more) mold parting surfaces or mold split lines. A stack mold does not require much more clamp force than a single phase mold because the projected part surface areas of the cavities on both sides of the center block cancel out each others force. A rule of thumb for a clamp force estimate is to take the projected part surface area times the melt pressure and multiply it by a factor of 1.1.

Stack Mold

16+16 stack mold hot half for high pressure PA6.6 thermal gate application with a 5 second cycle time

Without increasing the machine platen size you can double the amount of cavities producing parts. Most stack molds have an equal number of the same cavities in each parting surface. For example, a 2+2 cavity grid mold with an 8+8 hotrunner produces four identical parts per cycle.

Some stack molds have different cavities in each mold parting surface and they produce a family of parts per shot, each different in shape and size (such as this collapsible crate).

Injection Molded Collapsible Crate

Mold Flow Analysis

The floor and four side walls of this collapsible crate are produced in one shot. Fill, pack, cool and warp analysis accurately simulate the flow of the melt during the design phase of a project. This optimizes plastic part quality and the production output by improving cycle time.

Some stack molds can also handle multi-material injection, different materials such as hard/soft combination or multi-color molding. Three level stack molds are not as common as two level stack molds.

When the stack mold opens for the ejection of parts, both mold parting surfaces open (in most cases simultaneously) by means of a mechanical connection between the mold center block, the fixed halves of the mold and the moving half of the mold. The mechanical connection which moves the center block of the mold can be:

a) rack and gear operated motion system
b) harmonic lever system
c) hydraulic push-pull system or
d) helical gear system

Aside from the center block motion system, another key element of a stack mold is the hot runner system. The transfer of plastic melt from the machine barrel into the mold center block has traditionally been achieved by a heated sprue bar which leads the melt from the machine nozzle through the fixed mold half and through the first mold parting surface into the hot runner manifold, which is located in the center block of the stack mold.

The relatively simple sprue bar has the disadvantage of getting disengaged from the machine nozzle at each cycle when the mold opens for part ejection. The sprue bar may also become an obstacle during part removal, especially with robot take out/handling equipment because it always remains inside the first mold parting surface.

Mold cavities which by design cover the center surface of the molding area do not allow a center sprue bar so the hot runner has to by-pass the melt by leading it with an off-set manifold to the outside of the mold and from the side position through the first mold parting line into the center block for further melt distribution through the nozzles to the cavities.

Large Stack Mold

Hot half for a large stack mold 160 x 152 cm (63 x 60” ) 1+1 cavities for HDPE flame-retardant pallet with a part weight of 7.7 kg (17.5 lbs)


Learn more


Rheo-Pro® iVG™ Back-to-Back Stack Molds
Rheo-Pro® Valve Gate Controllers
Valve Gating
Family Molding




  1. Doubled Part Output
Instead of increasing the mold size by adding more cavities, a stack mold maintains mold size and machine size by adding a second layer of cavities parallel to the first layer. The fill, pack and cooling time remain the same for a stack mold and only the mold open and mold close time will add slightly to the cycle time.
  2. Reduced Part Price .
The part price is determined by the machine hour rate, which is directly related to the machine clamp tonnage. A stack mold requires only about half the clamp tonnage than a single-face mold with the same number of cavities.
  3. Efficiency and Improved Automation .
Stack molds can produce multi-component assemblies in one shot and in one machine using the same parameters. By comparison, single-face molds would require the production synchronization between multiple machines, complicating post-molding operations.

Gate-to-Gate Melt Transfer

Increasingly more popular is the melt transfer through the first mold parting surface without a sprue bar because it eliminates the sprue bar as an obstacle. The basic principle of a “sprue-bar-less” melt transfer is a nozzle-to-nozzle melt transfer.

As the mold parting surfaces close, two nozzle front ends establish a melt passage. These nozzles separate when the mold opens. A leak free connection or seal at 2000 bar or 30,000 psi is accomplished with the specially designed manifold system for stack molds from Mold Hotrunner Solutions®. During mold opening, the transfer nozzles provide a dry surface with absolutely no stringing or drooling of the melt.

A fully automated 24/7 molding operation with stack molds requires a functional melt transfer which operates over millions of cycles.

Sequentially controlled stack mold valve gate manifold with a four nozzle valve gate to valve gate melt transfer through the mold parting line featuring cooling free Black Box™ actuators

Generally, flat parts are more suitable for stack mold consideration than are deep core parts because of the machine opening stroke and the increased mold height of a stack mold. It is also important to check the shot capacity of the machine barrel ensuring that the injection unit can supply enough melt volume cycle for cycle and for all cavities.

MHS stack molds using Rheo-Pro® hot runner systems have a long track record for application success in thin wall packaging, caps and closures, containers and lids, collapsible crates, trays, panels, grills and large surface parts for appliances.

Tandem Molding
Tandem molding is a type of stack molding process in which the parts are injected in alternating cycles. While the molding machine opens to demold one parting line, the other is held together by a locking system. The technology can be employed in almost any standard injection molding machine and offers all of the benefits of stack molding as well as extended cooling times.

Flow Analysis

Fill analysis for Tandem molded washer components with metal insert molding, valve-to-valve melt transfer through part center hole. First part weight 3.6kg, second part 1.0kg

  Cycle 1
Part A Cooling
Part B Filling
Cycle 2
Part A Filling
Part B Cooling

Compact iVG™ Valve Gate Stack Molds
The Rheo-Pro® iVG™ is an internal actuated valve gate system with no external cyclinder and no elastomeric seals. It requires no cooling. iVG™ represents the latest and most innovative valve gate technology in the industry and is ideal for compact, reliable stack mold solutions.

16+16 Rheo-Pro® iVG™ back-to-back valve gate hot half featuring fully front accessible nozzles (with integrated actuators) for medical syringe plungers

Cooling-Free Valve Gating
Black Box™ and iVG™ hot runners represent a new generation of valve gate systems that does not require maintenance during the life of the mold. These systems eliminate all quality issues related to cooling water such as corrosion and contamination. Less plumbing also simplifies the design and assembly of the mold. New cooling-free technology can benefit all valve gate applications, including high temperature plastics such as PEEK, LCP, PSU, PEI, and PPS, known for their special properties. Processing these materials requires high melt temperatures of up to 450°C in the hot runner and mold temperatures of over 200°C. Black Box™ and iVG™ hot runner systems deliver uninterrupted performance under extreme conditions without cooling or wear. The result is a significant increase in reliability and mold uptime.


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